Geoscience Reference
In-Depth Information
rejected out of hand. But they were wrong. An obstinate set of prob-
lems makes interpreting fossil data so difficult that testing the
extinction half of the Alvarez theory proved much harder than any-
one could have anticipated. In the testing, however, so much was
learned that some paleontologists believe that their field is undergo-
ing a procedural if not a scientific revolution. For the first time, large
collections are being established with the specific aim of testing
whether extinction near a major geologic boundary was gradual or
sudden, and whether species that have always been thought to have
gone extinct at a boundary truly did so.
In order to understand how the second half of the Alvarez the-
ory was tested, it is important first to recognize some of the prob-
lems inherent in trying to read the fossil record. Common sense tells
us that to corroborate the extinction half of the theory, we need to
find two kinds of evidence: (a) that prior to the K-T boundary, most
species were not already going extinct for some other reason, and
(b) that the dinosaurs and others did not survive the K-T impact—
that their remains do not lie above the iridium layer. To test both
predictions, geologists needed to be able to pin down the exact point
in a sequence of rocks at which the extinction of a particular species
occurred. Can that be done?
G
APS
Darwin recognized, as noted in the epigraph that opens this chapter,
one insuperable problem with interpreting the fossil evidence: Ero-
sion has caused the geologic record to be riddled with missing rock
units. As shown in Figure 17, a missing unit can lead to the false con-
clusion that a fossil species became extinct earlier and more sud-
denly than it actually did. Therefore, unless there is independent
evidence that a geologic section contains no gaps, an apparently sud-
den extinction cannot be taken at face value. This works against the
FIGURE I
7 Erosion
distorts the geologic record.
[After Raup.
10
]
